1. Field
This invention relates to calorimetric or spectrometric devices for detecting materials in gaseous, liquid or solid samples by impingement of laser beams thereon to discern changes in the temperature of said fluid by absorption of energy from said laser beam by such materials sought to be identified, said materials selectively absorbing energy of different wavelengths.
2. Prior Art
The thermal lens effect results whenever a laser beam is impinged onto a material, e.g., a liquid, solid or gas, which is at least slightly transparent to said beam whereby a certain portion of the energy is absorbed, increasing the temperature of the material especially in the center of the beam. The temperature increase generally results in a lowering of the refractive index, producing a diverging lens effect which defocuses the beam.
The thermal lens effect, first reported by Gordon et al, J. Appl. Phys. 36, 3 (1965), is produced in an experimental arrangement similar to normal single beam absorption spectrometry. The major difference is that laser radiation passing through a sample is detected only at the center of the beam by restricting the field of view of the detector with a pinhole. The sample causes a loss of radiation from the beam center by thermal defocussing; that is, light absorbed by the sample is converted to heat by non-radiative relaxation and increases the temperature of the solvent by an amount which is greatest at the center of the beam. This temperature increase results in a lowering of the refractive index, producing a negative lens which defocusses the beam.
If the path from the laser to the sample is initially blocked and then opened with a shutter, the thermal lens takes a finite time to build up. A steady state condition is obtained when the rate of laser heating equals the rate of heat loss due to the thermal conductivity of the solvent and the finite temperature rise. The buildup of the lens can take place on time scales from tens of microseconds to hundreds of milliseconds depending on the thermal conductivity of the solvent and the radius of the laser beam through the sample.
The intensity measured at the beam center, I(t), will initially (t=0) reflect only the Beer's law response of the sample. After sufficient time, when a steady state temperature difference is reached, the intensity of the detector, I(oo), depends on the optical arrangement of the system. An optimum configuration which minimizes I(oo) is obtained when the sample is placed one confocal length beyond the beam waist formed by a long focal length lens. In this configuration, using TEM.sub.oo laser beam to probe a sample whose length, l, is sufficiently small (l&lt;&lt;2.pi.W.sub.o.sup.2 /.lambda., where w.sub.o is the beam waist, n is the refractive index and .lambda. is the laser wavelength), the following expression governs the initial and final intensities (2): ##EQU1## where P is the laser power in watts, dn/dT is the change in solvent refractive index with temperature (usually negative), A is the sample absorbance, .lambda. is the laser wavelength, k is the thermal conductivity in watts/cm K and E is the enhancement of this effect relative to Beer's law behavior. This expression assumes that all of the absorbed light is converted into heat. If the quantum yield of fluorescence is finite, then a correction term which includes the quantum yield and Stokes shift may be applied. The choice of solvent for a determination governs the enhancement, E, that one realizes for a particular laser power. Table I lists several solvents, their pertinent thermo-optical properties and the enhancement per unit laser power (in mW) taking as the wavelength .lambda.=632.8 nm from the visible He:Ne laser transition.
TABLE 1 ______________________________________ Thermo-optical Properties of Solvents For Thermal Lens Measurements.sup.a k 10.sup.4 . dn/dT E/P.sup.b Solvent (mW/cm .degree. K.) (.degree.K..sup.-1) (mW.sup.-1) ______________________________________ CCl.sub.4 1.02 5.8 8.93 Benzene 1.44 6.4 7.02 Acetone 1.60 5.0 4.97 Methanol 2.01 3.9 3.06 Water 6.11 0.8 0.21 ______________________________________ .sup.a Data taken from Solimini. J. Appl. Physics 37, 3314 (1966). .sup.b Enhancement per unit laser power in milliwatts, .lambda. = 632.8 nm.
To minimize the time constant of the effect and while maximizing the response, the thermal lens is generally measured in an optical configuration shown in FIG. 1. The laser beam of divergence, .theta., enters from the left through a lens, 11, having focal length, f. This produces a minimum beam radius, w.sub.o .ltoreq.f.multidot..theta..
The sample is placed at a position, 13, relative to the beam waist. The influence of the position has been derived by Hu and Whinnery, Applied Optics 12, 72 (1973), for a thin sample such that w does not change over the cell path. ##EQU2## The expression inside the brackets maximizes to a value of 1 when w=w.sub.o .sqroot.2 which reduced Equation 2 to Equation 1. To obtain the thermal lens effect versus a more useful experimental parameter, z, the expression, ##EQU3## is substituted for w.sup.2 in Equation (2) to give ##EQU4## where z.sub.c is the confocal distance, z.sub.c =.pi.w.sub.o.sup.2 /.lambda.. The expression inside the brackets showing the all position dependence is plotted in FIG. 2 and has maxima and minima when z=.+-.z.sub.c.
U.S. Pat. No. 4,048,499 to Kreuzer employs an infrared laser beam of predetermined wavelength to produce heating of a sample, e.g., a liquid, which contains components which absorb energy from the laser beam. A thermal detector, such as a thermistor, is disposed in thermal energy exchanging relocation with the liquid sample which experiences a limited temperature increase due to energy absorption by components therein. The device of Kreuzer is limited in its sensitivity by the effectiveness of the thermistor and its failure to discriminate between absorption by components to be detected and background absorption by the liquid carrying medium, cell walls, etc.